The present invention relates to novel glass compositions, and more particularly to glasses formed of metal fluorides and exhibiting desirable optical properties and enhanced stability.
Although most of the glasses in commercial use are characterizable as oxide glasses, i.e., formed by the fusion of mixtures of oxides, glasses based on metal halides are also well known. Perhaps the best known glass forming metal halide is BeF.sub.2, which forms a relatively stable glass when fused alone. But BeF.sub.2 itself, which is also a component of many of the more stable halide glasses, is both hygroscopic and highly toxic.
Fluoride compositions providing alternatives to the use of BeF.sub.2 include those described, for example, in U.S. Pat. No. 4,141,741. That patent describes glasses based on ZrF.sub.4 and comprising additional constituents such as BaF.sub.2 and ThF.sub.4 which exhibit the desirable infrared transparency characteristic of many fluoride glasses and which are, in addition, non-hygroscopic. Further, U.S. Pat. No. 4,308,066 describes fluoride glasses based on ZrF.sub.4 and/or AlF.sub.3 which will form glasses if rapidly quenched. These glasses include, as further constituents, fluorides such as CaF.sub.2, SrF.sub.2, BaF.sub.2 and/or PbF.sub.2. All of these glasses generally share a short working range and are prone to devitrification.
Additional fluoride glasses comprising ZrF.sub.4 have been described by M. Matecki et al, Mat. Res. Bull., 17, pages 1035-1043 (1982). These are ThF.sub.4 -ZrF.sub.4 glasses containing fluorides such as LaF.sub.3, YF.sub.3, CdF.sub.3, LuF.sub.3, and ScF.sub.3. The glasses are reportedly relatively stable, exhibiting glass transition temperatures of 460.degree.-515.degree. C., and exhibit good infrared transparency.
Glasses based on fluorides other than ZrF.sub.4 have also been reported. Hence, S. Shibata et al, Mat. Res. Bull., 15, pages 129-137 (1980), describe a family of PbF.sub.2 -based fluoride compositions which will form glasses if rapidly quenched. Further, M. Matecki et al, Mat. Res. Bull., 17, pages 1275-1281 (1982) have reported CdF.sub.2 -based glasses containing BaF.sub.2 or a combination of BaF.sub.2 and ZnF.sub.2 which can form thin infrared transmitting glass if rapidly quenched. Optional constituents of these glasses include AlF.sub.3, YbF.sub.3, ThF.sub.4, and/or alkali metal fluorides.
More recently, stable halide glasses have been reported in the CdF.sub.2 -AlF.sub.3 -PbF.sub.2 system. Hence U.S. Pat. No. 4,537,864 reports CdF.sub.2 -AlF.sub.3 -PbF.sub.2 glasses, preferably also including LiF, which exhibit good glass stability and relatively low glass transition temperatures. These glasses may contain optional constituents such as NaF, ZnF.sub.2, CaF.sub.2, SrF.sub.2, BaF.sub.2, YbF.sub.3 and KF; however, only YbF.sub.3 and KF may be introduced in significant proportions without some destabilization of the glass.
Glasses such as reported in U.S. 4,537,864 are of particular interest where high purity is demanded in the halide glass, because each of the constituents of the base glass is potentially vapor transportable. Hence impurities such as the transition metals and other species, which undesirably lower the transparency of the glass, can be largely excluded by vapor phase processing. However, the region of best glass stability in this system is not large, and composition modifications which could improve the stability of the glass and/or the transparency thereof would be desirable.